1. Field of the Invention
This invention relates to a high-voltage transformer such as an ignition coil for internal combustion engines in which a primary coil, a secondary coil and an iron core are electrically insulated from each other and firmly installed in and secured to a coil case by a resin impregnated therein.
2. Description of the Prior Art
FIG. 1 is a plan view showing a conventional high-voltage transformer for internal combustion engines, and FIG. 2 is a cross sectional view taken along line II--II of FIG. 1. In these figures, the high-voltage transformer 1 in the form of an ignition coil comprises a generally cylindrical coil case 2 formed of a synthetic resin, a primary coil 3 in the coil case 2, a secondary coil 4 disposed in the coil case 2 so as to surround the primary coil 3, a cylindrical sleeve 5 disposed in and fixedly mounted on the coil case 2 substantially at the center thereof for receiving an unillustrated rotary shaft of a distributor, and an iron core 6 disposed in the coil case 2 around the sleeve 5 so as to surround the primary coil 3 and the secondary coil 4.
The iron core 6 comprises an annular inner or central leg portion 601 disposed around the cylindrical sleeve 5 and radially inside the primary coil 3, four planar outer leg portions 602 disposed radially outside the secondary coil 4, a pair of first (or lower) and second (or upper) cross-shaped arm portions 603 and 604 interconnecting the inner and outer leg portions 601 and 602 for forming a closed magnetic path which passes through the primary and secondary coils 3, 4 when these coils are energized. The annular inner leg portion 601 is in contact at its opposite ends with the inner surfaces of the lower and upper arm portions 603, 604. The outer leg portions 602 are slightly shorter than the inner leg portion 601 so that they are in contact at their lower end with the cross-shaped lower arm portion 603 but spaced from the upper arm portion 604 with a limited gap 605 formed therebetween.
A resin 7 is filled into the coil case 2 and impregnated into the spaces between the coils 3, 4 and the iron core 6 for electrically insulating the coils 3, 4 and the iron core 6 from each other as well as for firmly securing or bonding them to the coil case 2. In this case, the lower arm portion 603 of the iron core 6 is moulded integrally with or otherwise firmly connected with the coil case 2, and it is exposed to the outside of the coil case 2 for dissipating heat which is generated during moulding of the coil case 2.
With the conventional high-voltage transformer 1 as constructed above, the lower cross-shaped arm portion 603 of the iron core 6 is first disposed in and integrally moulded or otherwise firmly connected with the coil case 2, and then the annular inner leg portion 601, the primary coil 3, the secondary coil 4 and the outer leg portion 602 are disposed in the coil case 2. Thereafter, the upper arm portion 604 of the iron core 6 is placed on the inner and outer leg portions 601, 602, and the resin 7 in a molten state is filled into the coil case 2 up to a predetermined level, impregnated into the spaces between the above members and solidified to firmly install these members in the coil case 2. In this manner, the resin 7 thus impregnated serves not only for securing the members to the coil case 2 but also for improving the electrical insulation therebetween.
In general, the high-voltage transformer 1 for internal combustion engines is frequently subject to temperature changes which arise, for example, between day and night, between seasons or the like. Further, the transformer 1 is repeatedly subject to great head shocks from an engine during the travel of a vehicle on which the transformer 1 and the engine are installed. As a result, due to the fact that the resin 7 has a coefficient of thermal expansion substantially different from that of the iron core 6, the resin 7 is liable to be peeled off at the contacting surfaces between the resin 7 and the side surfaces of the upper arm portion 604 of the iron core 6 or cracks will arise particularly at the inside corners of the upper arm portion 604. The peeling off of the resin 7 or the cracks thus created therein would deteriorate the electrical insulation of the resin 7, causing a leakage or reduction of high voltage at the secondary coil 4. Accordingly, there arises a problem in that a high voltage required for ignition plugs of the engine can not be obtained, resulting in engine trouble.